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Understanding of the underlying causes of spatial variation in exchange of carbon and water 25 vapor fluxes between grasslands and the atmosphere is crucial for accurate estimates of regional 26 and global carbon and water budgets, and for predicting the impact of climate change on 27 biosphere-atmosphere feedbacks of grasslands. We used ground-based eddy flux and 28 meteorological data, and the Moderate Resolution Imaging Spectroradiometer (MODIS) 29 enhanced vegetation index (EVI) from 12 grasslands across the United States to examine the 30 spatial variability in carbon and water vapor fluxes and to evaluate the biophysical controls on 31 the spatial patterns of fluxes. Precipitation was strongly associated with spatial and temporal 32 variability in carbon and water vapor fluxes and vegetation productivity. Grasslands with annual 33 average precipitation < 600 mm generally had neutral annual carbon balance or emitted small 34 amount of carbon to the atmosphere. Despite strong coupling between gross primary production 35 (GPP) and evapotranspiration (ET) across study sites, GPP showed larger spatial variation than 36 ET, and EVI had a greater effect on GPP than on ET. Consequently, large spatial variation in 37 ecosystem water use efficiency (EWUE = annual GPP/ET; varying from 0.67 ± 0.55 to 2.52 ± 38 0.52 g C mm-1 ET) was observed. Greater reduction in GPP than ET at high air temperature and 39 vapor pressure deficit caused a reduction in EWUE in dry years, indicating a response which is 40 opposite than what has been reported for forests. Our results show that spatial and temporal 41 variations in ecosystem carbon uptake, ET, and water use efficiency of grasslands were strongly 42 associated with canopy greenness and coverage, as indicated by EVI.

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Burning, grazing, and baling (hay harvesting) are common management practices for tallgrass pasture. To develop and adopt sustainable management practices, it is essential to better understand and quantify the impacts of management practices on plant phenology and carbon fluxes. In this study, we combined multiple data sources, including in-situ PhenoCam digital images, eddy covariance data, and satellite data (Landsat and Moderate Resolution Imaging Spectroradiometer (MODIS)) to examine the impacts of burning, baling, and grazing on canopy dynamics, plant phenology, and carbon fluxes in a tallgrass pasture in El Reno, Oklahoma in 2014. Landsat images were used to assess the baling area and the trajectory of vegetation recovery. MODIS vegetation indices (VIs) were used in the Vegetation Photosynthesis Model (VPM) to estimate gross primary production (GPP VPM) at a MODIS pixel for the flux tower (baled) site. For comparison between baled and unbaled conditions, we used MODIS VIs for a neighbor MODIS pixel (unbaled) and ran VPM. Daily PhenoCam images and green chromatic coordinate (GCC) tracked canopy dynamics and plant phenology well. The grassland greened up immediately after burning in April. GCC values showed two peaks with the similar magnitude because of quick recovery of grassland after baling. Satellite-derived VIs and GPP VPM showed that the pasture recovered in one month after baling. The GPP VPM matched well (R 2 = 0.89) with the eddy covariance-derived GPP (GPP EC). Grazing in the late growing season did not influence plant phenology (VIs and GCC) and carbon uptake (GPP) as plants were in the late growing stage. Neither did it affect GPP differently in those two conditions because of even grazing intensity. The reduction in GPP after baling was compensated by higher GPP after large rain events in late July and early September, causing little seasonal differences in GPP (-0.002 g C m-2 day-1) between the baled and unbaled conditions. Interactions of different management 3 practices with climate make it complicated to understand the impacts of different management practices on carbon dynamics and plant phenology. Thus, it is necessary to further investigate the responses of tallgrass pastures to different management practices under different climate regimes at multiple temporal and spatial scales.